Terraforming is the process of manipulating "the atmosphere of a planet to convert it to a sustained oxygen environment, making it habitable for humans." 
The History of TerraformingEdit
The first effort to successfully terraform a planet began on Mars in 2120. It was a joint effort on behalf of "the governments of the world."  In 2125 the effort had given Mars an atmosphere suitable for human use, but two days before declaring it a victory, tragedy struck. A "chemical miscalculation in the planetary atmospheric processors made the new atmosphere unstable" and led to it being unable to "stick" to the surface of Mars.  Unfortunately, the people of Mars had already stopped using breathing apparatuses, the loss of atmosphere happened "so fast that no one was able to sound an alarm or seal the ventilations of the various installations around the planet."  4,876 people died.  The event is referred to as The Great Mars Tragedy.  The project was completed successfully in 2157.